Analogue Recorders - Functioning

6.2.1. Functioning principle

The aim is to store information which will in some way represent the sound waveform and will allow it to be reproduced. Let's suppose that the wave we need to store is once again, our good old friend the sinusoid, always bearing in mind that our example can be quite effortlessly be extended to more complex waveforms. We will imagine to have already picked up the waveform with our microphone which converted it into an electrical signal. The recorder's heads contain an inductance
[Inductor
]
through which the current coming from the microphone flows. The magnetic ribbon is made up of various different materials which we'll shortly take a look at. What we are interested in right now, is one particular layer of the ribbon which is made up of thousands of magnetic particles, each of which is polar, in other words, with a negative and a positive pole. When the ribbon rolls over the head, the latter, through its inductance, polarizes the particles, in other words, it moves them in the direction dictated by the current's polarity.

In the following figure we can see how for the sinusoidal signal, the magnetic particles are forced to turn in a specific direction during the positive semiwave and in the exact opposite direction during the negative semiwave.

Magnetization of a magnetic ribbon

So the disposition of the particles on the magnetized ribbon represents the information we need to then reconstruct the original signal. Then, during the reproduction phase, another head of a different kind, makes contact with the magnetized ribbon and induces a current into the inductance contained in the head. This small inducted current, when appropriately amplified by a loudspeaker, reproduces the wave which had been stored in the recording phase.